Soybean Disease Update: July 22, 2017 (UPDATED 7/23/2017)

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 22, 2017 06:38

Soybean Disease Update: July 22, 2017 (UPDATED 7/23/2017)

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Several foliar diseases have increased in incidence throughout MS over the past several weeks. Root diseases, such as sudden death syndrome, taproot decline and one field of stem canker continue to be observed in the production system.  The current foliar disease situation is outlined below.

Frogeye leaf spot as observed in the upper most canopy.

Frogeye leaf spot

The early-planted soybean crop has been observed to contain little-to-no frogeye. However, over the past two weeks, frogeye has increased in some field situations due to the conducive environment and heavy dews in some geographies.  Later planted soybean fields should be scouted for the presence of frogeye.  Frogeye generally occurs on the newest leaf tissue in the upper plant canopy.  Do not confuse frogeye leaf spot with target spot.  Some target spot lesions can appear extremely similar to frogeye leaf spot.  To confirm the presence of frogeye, turn the leaf over and observe the lesion for the presence of the fungus that causes frogeye leaf spot.  Target spot will not produce fungal fruiting bodies on the underside of the leaf.  In addition, frogeye lesions tend to be more tan than dark brown in the center and the margins of the lesion tend to be maroon with no distinct yellow halo.  Scout frogeye leaf spot susceptible soybean varieties more regularly once plants reach reproductive growth stages.

 

 

Septoria brown spot as observed in the middle canopy.

Septoria brown spot

Septoria brown spot is one of the most common lower canopy diseases in the MS soybean production system. Almost annually I receive calls regarding perceived yield losses as a result of Septoria brown spot.  Diseases that stay in the lower canopy likely do not contribute to losses.  Fields with a history of soybean monoculture commonly contain brown spot on vegetative soybean plants.  In some situations, brown spot can move into the upper canopy; however, this is a rare occurrence.  Automatic fungicide applications at the more normal R3/R4 timing likely contribute some benefit to keeping brown spot low in the canopy.  But, with that said, I suspect there is widespread resistance within this particular fungal population to the strobilurin (or quinone outside inhibitor fungicides) active ingredients.  Field situations that have been observed to contain brown spot in the upper canopy in the past have generally suffered from another stress.  Be mindful that brown spot can be confused with several other diseases, particular bacterial blight this season due to the heavy dews.  In general, brown spot lesions are maroon, small in size and generally less than 1/16th of an inch with a yellow halo apparent when occurring in the middle canopy.  When the disease becomes severe the lesions can coalesce and cover a larger surface area.

 

Soybean rust

To date (July 23, 2017), soybean rust has been confirmed in five total counties in south MS on kudzu (Adams, Amite, Stone, Wilkinson) and one county in a soybean sentinel plot (Jackson). Given the temperatures over the past two weeks I suspect that soybean rust will not make a large leap into the soybean producing areas of the state.  The fungus that causes soybean rust is not active when temperatures reach the mid-90s as they have for the past two weeks.  However, soybean sentinel plots in the southern part of the state continue to be monitored for the occurrence of soybean rust.  So far, managing soybean rust has been easy since few acres have been observed to be infected with a severe level of soybean rust at early growth stages.  To stay current on the soybean rust situation as it continues to develop follow me on Twitter (@baldpathologist), continue to monitor the Mississippi Crop Situation Blog and also follow the ipmPIPE at http://sbrusa.net/cgi-bin/sbr/public.cgi for the most current map.

Target spot as observed in the mid-canopy.

Target spot

As compared to the 2016 season, target spot is not near as severe. However, some isolated cases of severe target spot have occurred on some of the super susceptible varieties that were identified last year.  Target spot is one of the most common diseases in the lower-to-middle canopy on an annual basis.  Environment is one of the most important ingredients for target spot to become yield limiting.  Fungicides may be beneficial at maintaining foliage on the soybean plant; however, once the disease has started and moved into the middle canopy it may be difficult to stop the development of the disease especially if the environment switches back to a conducive situation.  I am convinced that warm, sunny weather will be most beneficial with this particular disease.  However, be mindful that some varieties appear to contain substantial target spot again this season.  Some of this could be attributed to the number of acres that contain the particular varieties, but I suspect the situation is two-fold: some of the newer varieties may in fact be a bit more susceptible to target spot and the environment this season has been cooler and wetter than normal to this point which has allowed the disease to increase in severity.

I rated soybean fungicide efficacy trial plots on Tuesday in Stoneville. The majority of the field received applications at the R3 growth stage.  Out of 700 efficacy plots (with a small number receiving an R5 application) defoliation in the lower-to-middle canopy ranged from 15-50% regardless of fungicide product (one disclaimer, I do not look at what plot received which chemical until the end of the season).  I do know that some nontreated plots were observed to contain substantially more defoliation, on the order of 50%; however, given the number of plots observed, and the total number of tests in the field (10 tests) some of the treated plots suffered just as much defoliation.  I could not visually tell the differences between treated and nontreated plots based on the presence of the disease or defoliation.  With that in mind, I think an R3 application was able to maintain foliage in the lower-to-middle canopy and I do not think that a second fungicide application would be warranted at a later growth stage.  Even though the variety planted is more susceptible to frogeye, severe target spot was observed in fields in Stoneville as well as Starkville in this particular variety during the 2016 season.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 22, 2017 06:38
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